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2 stroke engine trouble shooting

auntiebling

auntiebling

megalomaniacal troglodyte
Super Moderator
Joined
Apr 27, 2004
Location
atop the parapet
Moto(s)
.
This is regarding a Comer 50 cc goakrt engine, apparently a piston port setup.

-i know it has spark
-it has enough compression to push my finger out of the way when I cover the spark plug hole, pushing as hard as I can while pulling the string
-if I spray starter fluid in the intake, it will cough
low-carb jet and emulsion tube are clear, compressed air passes through and the carb is NEW
-dellorto 1412 baby flat slide carb

BUT

It never seems to pull enough air through the intake (none so far as I can tell) to get the gas to flow out of the float bowl and into the carb throat.

I know very little about actually working on 2 strokes. It's weird to me that I can't get any kind of vacuum signal at the intake manifold but it has compression.

Is the crank seal totally gone?

Before y'all get all "why isn't this a motorcycle" shush. I could put it in a bike frame. It happens to be in a race kart my kiddo was given and he badly wants to go faster than his now too small power wheels will go.
 
what's the condition of the exhaust? does the engine have a lot of hours on it? If so, pull the exhaust and check for carbon build up in the exhaust port. Hard to advise without more details
 
Exhaust: looks fine. What am I looking for? It's a stock box looking thing. I can blow air through it easily with my lungs

Hours: no idea. Probably lots. Guy I got it from couldn't solve this issue before his kids lost interest. Prior to that it was a winning ride at the hands of a kid that now allegedly races sprint cars (kart is old)
 
Does it have one of those reed valve things?
 
really need to do a compression test with a guage to verify before going insane trying to solve the problem. can you inspect the bore condition? Sounds like it's worn out
 
Will see of I can find my tester. I can Google of course but what's the procedure on a 2 stroke pull start? Leak down moreso, no valves how does that work?
 
We used to call that "the rule of thumb". :laughing That is , if it will blow your thumb off, it has enough to run. You should test the motor for leak down. Not the standard 4 stroke type, but block off the exhaust and intake, making a method of putting air into the crankcase at about 6 to ten psi. Let stand and re check pressure. Crank seals should hold pressure, if not it's an air leak into you're mixture. Possible leaks are: crank seals, seal between the case joints, leaking base gaskets, and anywhere air can enter the crankcase. If it will hold pressure, that's not your problem. If it leaks, check all of the above with pressure and soapy water. A method of pressurizing is to put a schrader valve into a plate that you use to block the intake, or the exhaust. The real way involves use a crankcase pressure test kit that has a squeeze bulb, and a gauge . You can sometimes plug the ex or the in with large rubber plugs. Don't go overboard on the pressure, as you can blow the seals. Best to use a bicycle pump, or other hand pump. If no gauge available, put in a few pumps, let it sit, and press on the schrader valve and listen for a hiss. There must be a seal on case for a 2 stroke to function.
 
Sounds like I have a couple adapters to make. Do I draw them up for the machinist at work? No. That would be unethical

Piston has to be at TDC right? Why wouldn't it?
 
I would start with a standard compression check of the motor. Even if your crankcase has a small leak putting some fuel in the sparkplug hole should get the motor running.

3 things are needed: fuel, spark, compression.

If your compression test shows enough pressure the next thing I would look at is the strength of the spark (should be a nice fat blue one). If that checks out determining when the spark is firing is the last thing on the list.

I am not familiar with the Comer motors so I am not sure if it runs CDI or a points ignition.
If it's points - The stator plate should have slots to allow the spark timing to be adjusted. If it's firing way too soon or way too late it would prevent proper ignition from taking place. There are tons of resources online for measuring this with a multimeter.
CDI - Small displacement CDI boxes are notoriously cheaply made. You might be able to see when it fires with a basic timing light. You should be able to get the motor spinning fast enough using a drill with a socket the same size as the flywheel nut.

Those Dellorto SHA carbs are about the most simple things out there. Only adjustment is the idle screw that slightly raises or lowers the slide.


Based on the photos I can find that is a piston port motor. Crankcase leak down test should be done with the piston at TDC to ensure the intake port is open to the crankcase.
 
Negative. Leak test should be done with the piston below tdc. You want to check the integrity of the crankcase and it's seals, not the piston rings. A compression test will tell you about the rings.

Although with 10 or so psi, the ring gap would let the pressure into the case. I've done probably 25 or so 2 stroke leakdowns. Maybe more, because back in the day, most of the Kawasakis were 2 stroke. The crankcase is part of the intake tract. You don't need a machinist to make block off plates. If its bolts, just make a piece of aluminum with a car air valve in it, and block off the other, and use a bicycle pump. It's not rocket science, you just need to know if the case will hold air.
 
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Here I am again! Thinking about what motor you have, I wonder if it has a pumper carb on it like a chain saw. A good way to test if your problem is fuel delivery, is put some gas down the spark plug hole ( just a little, you don't want to flood the plug). If it barks, it's probably fuel delivery. Pumper carbs rely on engine vac/pressure to operate a diaphram on the carb. It's a little vacuum operated fuel pump. If the diaphragm is torn, or not seated, no fuel is delivered. Try some gas, not starting fluid.
 
Tha is for the advice all, it'll be a week at least till I can work through this all, but I can tell you it's not a pumper carb. Venturi only pitting fuel into the carb throat.
 
Demoni said: "3 things are needed: fuel, spark, compression."
Actually 4. There's also timing. Lots of 2 strokes have pressed together cranks. If your crank has slipped timing, you will chase your tail forever. I'm not saying this IS your problem, but you need to be aware of this issue as well.
 
Lots of good advice. My basics whenever I start working on a two stroke is leakdown test, compression, test, then spark, and then fuel. Timing is important too (sometimes the key on the crank will shear off throwing timing off.)

I dont know small cart motors, but do a lot of two stroke dirtbike engines.

You can easily make a two stroke leakdown tester from parts found at Home Depot in the plumbing department for under $30. Rubber pipe end fitting with hose clamp to block off exhaust and a PVC fitting that fits into your intake boot. Run a hose from the intake boot to a low pressure gas gauge fitting with a schrader valve (so you can introduce air pressure.)

I like to do 8lbs with no pressure drop for 10 minutes. Motors I build can usually hold pressure for hours.

Anyways, just my contribution. Here is the one I made, cheap and easy.
 

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just some various pictures
muffler
carb
exhaust port
carb again
 

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